Editing CNO cycle (section)

{{short description|Catalysed fusion reactions by which stars convert hydrogen to helium}}
{{Use dmy dates|date=August 2021}}
[[File:Nuclear energy generation.svg|right|upright=1.5|thumb|300px|[[Logarithm]] of the relative energy output (ε) of [[Proton–proton chain reaction|proton–proton]] (p–p), CNO, and [[Triple-alpha process|triple-α]] fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the p–p and CNO processes within a star.]]

In [[astrophysics]], the '''carbon–nitrogen–oxygen''' ('''CNO''') '''cycle''', sometimes called '''Bethe–Weizsäcker cycle''', after [[Hans Albrecht Bethe]] and [[Carl Friedrich von Weizsäcker]], is one of the two known sets of [[nuclear fusion|fusion]] [[nuclear reaction|reactions]] by which [[star]]s convert [[hydrogen]] to [[helium]], the other being the [[proton–proton chain reaction]] (p–p cycle), which is more efficient at the [[Sun]]'s core temperature. The CNO cycle is hypothesized to be dominant in stars that are more than 1.3&nbsp;times as [[Solar mass|massive as the Sun]].<ref name=salaris_cassini2005/>

Unlike the proton-proton reaction, which consumes all its constituents, the CNO cycle is a [[catalytic cycle]]. In the CNO cycle, four [[proton]]s fuse, using [[carbon]], [[nitrogen]], and [[oxygen]] isotopes as catalysts, each of which is consumed at one step of the CNO cycle, but re-generated in a later step. The end product is one [[alpha particle]] (a [[stable nuclide|stable]] [[helium]] nucleus), two [[positron]]s, and two [[electron neutrino]]s.

There are various alternative paths and catalysts involved in the CNO cycles, but all these cycles have the same net result:
:4 {{nuclide|Hydrogen|1}} &nbsp; + &nbsp; 2 {{SubatomicParticle|Electron}}
:: &nbsp; → &nbsp; {{nuclide|Helium|4}} &nbsp; + &nbsp; {{math|2 {{SubatomicParticle|Positron}} &nbsp; + &nbsp; 2 {{SubatomicParticle|Electron}} &nbsp; + &nbsp; 2 {{SubatomicParticle|Electron neutrino}} &nbsp; + &nbsp; 3 {{SubatomicParticle|Gamma}}}} &nbsp; + &nbsp; {{val|24.7|ul=MeV}}
:: &nbsp; → &nbsp; {{nuclide|Helium|4}} &nbsp; + &nbsp; {{math|2 {{SubatomicParticle|Electron neutrino}} &nbsp; + &nbsp; 7 {{SubatomicParticle|Gamma}}}} &nbsp; + &nbsp; {{val|26.7|u=MeV}}

The positrons will almost instantly [[Electron–positron annihilation|annihilate with electrons]], releasing energy in the form of [[gamma ray]]s. The neutrinos escape from the star carrying away some energy.<ref name="BOREXINO">{{cite journal
 |collaboration=The BOREXINO collaboration
 |date=25 June 2020
 |title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun
 |arxiv=2006.15115
 |language=en|last1=Agostini
 |first1=M.
 |last2=Altenmüller
 |first2=K.
 |last3=Appel
 |first3=S.
 |last4=Atroshchenko
 |first4=V.
 |last5=Bagdasarian
 |first5=Z.
 |last6=Basilico
 |first6=D.
 |last7=Bellini
 |first7=G.
 |last8=Benziger
 |first8=J.
 |last9=Biondi
 |first9=R.
 |last10=Bravo
 |first10=D.
 |last11=Caccianiga
 |first11=B.
 |last12=Calaprice
 |first12=F.
 |last13=Caminata
 |first13=A.
 |last14=Cavalcante
 |first14=P.
 |last15=Chepurnov
 |first15=A.
 |last16=D'Angelo
 |first16=D.
 |last17=Davini
 |first17=S.
 |last18=Derbin
 |first18=A.
 |last19=Di Giacinto
 |first19=A.
 |last20=Di Marcello
 |first20=V.
 |last21=Ding
 |first21=X. F.
 |last22=Di Ludovico
 |first22=A.
 |last23=Di Noto
 |first23=L.
 |last24=Drachnev
 |first24=I.
 |last25=Formozov
 |first25=A.
 |last26=Franco
 |first26=D.
 |last27=Galbiati
 |first27=C.
 |last28=Ghiano
 |first28=C.
 |last29=Giammarchi
 |first29=M.
 |last30=Goretti
 |first30=A.
 |journal=Nature
 |volume=587
 |issue=7835
 |pages=577–582
 |doi=10.1038/s41586-020-2934-0
 |pmid=33239797
 |bibcode=2020Natur.587..577B
 |s2cid=227174644
 |display-authors=2
 }}</ref> One nucleus goes on to become carbon, nitrogen, and oxygen isotopes through a number of transformations in a repeating cycle.

[[Image:CNO Cycle.svg|300px|right|thumbnail|Overview of the CNO-I Cycle]]
The proton–proton chain is more prominent in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reaction starts at temperatures around {{val|4|e=6|ul=K}}<ref>{{cite book
 | last1=Reid    | first1=I. Neill
 | last2=Hawley  | first2=Suzanne L.
 | year=2005
 | chapter=The structure, formation, and evolution of low-mass stars and brown dwarfs &ndash; Energy generation
 | title=New Light on Dark Stars: Red dwarfs, low-mass stars, brown dwarfs
 | edition=2nd | pages=108–111
 | series=Springer-Praxis Books in Astrophysics and Astronomy
 | publisher=[[Springer Science & Business Media]]
 | isbn=3-540-25124-3
 | chapter-url=https://books.google.com/books?id=o7pe7Fp4JaAC&pg=PA108 }}</ref> (4&nbsp;megakelvin), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts at approximately {{val|15|e=6|ul=K}}, but its energy output rises much more rapidly with increasing temperatures<ref name=salaris_cassini2005>{{cite book
 | first1=Maurizio | last1=Salaris
 | first2=Santi    | last2=Cassisi
 | year=2005
 | title=Evolution of Stars and Stellar Populations
 | pages=[https://archive.org/details/evolutionofstars0000sala/page/119 119]–121
 | publisher=[[John Wiley and Sons]]
 | isbn=0-470-09220-3
 | url=https://archive.org/details/evolutionofstars0000sala
 | url-access=registration
}}</ref> so that it becomes the dominant source of energy at approximately {{val|17|e=6|ul=K}}.<ref>
{{cite journal
 | last1=Schuler |first1=S.C.
 | last2=King    |first2=J.R.
 | last3=The     |first3=L.-S.
 | year=2009
 | title=Stellar Nucleosynthesis in the Hyades open cluster
 | journal=[[The Astrophysical Journal]]
 | volume=701  | issue=1  | pages=837–849
 | arxiv=0906.4812  | bibcode=2009ApJ...701..837S
 | doi=10.1088/0004-637X/701/1/837  |s2cid=10626836
 }}</ref>

The Sun has a [[solar core|core]] temperature of around {{val|15.7|e=6|ul=K}}, and only {{val|1.7|s=%}} of {{SimpleNuclide|Helium|4|link=yes}} nuclei produced in the Sun are
born in the CNO cycle.

The [[#CNO-I|CNO-I]] process was independently proposed by [[Carl Friedrich von Weizsäcker|Carl von&nbsp;Weizsäcker]]<ref name = vonWeizsäcker-1>
{{cite journal
 |last=von&nbsp;Weizsäcker  |first=Carl F.  |author-link=Carl Friedrich von Weizsäcker
 |title=Über Elementumwandlungen in Innern der Sterne&nbsp;I
 |trans-title= On transformations of elements in the interiors of stars&nbsp;I
 |journal=[[Physikalische Zeitschrift]]
 |volume=38  |year=1937  |pages=176–191
}}</ref><ref name = vonWeizsäcker-2>
{{cite journal
 |last=von&nbsp;Weizsäcker  |first=Carl F.  |author-link=Carl Friedrich von Weizsäcker
 |title=Über Elementumwandlungen in Innern der Sterne&nbsp;II
 |trans-title= On transformations of elements in the interiors of stars&nbsp;II
 |journal=[[Physikalische Zeitschrift]]
 |volume=39 |year=1938 |pages=633–646
}}</ref> and [[Hans Bethe]]<ref name = Bethe-1939-a>
{{cite journal
 |last = Bethe  |first = Hans A.  |author-link = Hans Bethe
 |year = 1939
 |title = Energy Production in Stars
 |journal = [[Physical Review]]
 |volume = 55  |issue = 1  |pages = 541–7 |doi = 10.1103/PhysRev.55.103
 |pmid = 17835673  |doi-access = free
 |bibcode = 1939PhRv...55..103B
}}</ref><ref name = Bethe-1939-b/> in the late 1930s.

The first reports of the experimental detection of the neutrinos produced by the CNO cycle in the Sun were published in 2020 by the [[Borexino|BOREXINO]] collaboration. This was also the first experimental confirmation that the Sun had a CNO cycle, that the proposed magnitude of the cycle was accurate, and that von Weizsäcker and Bethe were correct.<ref name="BOREXINO"/><ref>{{Cite journal|last1=Agostini|first1=M.|last2=Altenmüller|first2=K.|last3=Appel|first3=S.|last4=Atroshchenko|first4=V.|last5=Bagdasarian|first5=Z.|last6=Basilico|first6=D.|last7=Bellini|first7=G.|last8=Benziger|first8=J.|last9=Biondi|first9=R.|last10=Bravo|first10=D.|last11=Caccianiga|first11=B.|date=25 November 2020|title=Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun|url=https://www.nature.com/articles/s41586-020-2934-0|journal=Nature|language=en|volume=587|issue=7835|pages=577–582|doi=10.1038/s41586-020-2934-0|pmid=33239797|issn=1476-4687|quote=This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent;|arxiv=2006.15115|bibcode=2020Natur.587..577B|s2cid=227174644}}</ref><ref>{{Cite web|title=Neutrinos yield first experimental evidence of catalyzed fusion dominant in many stars|url=https://phys.org/news/2020-11-neutrinos-yield-experimental-evidence-catalyzed.html|access-date=2020-11-26|website=phys.org|language=en|quote=Pocar points out, "Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work."}}</ref>